Press spinach between layers of paper towels, squeezing until barely moist. Saute onion in 3 tablespoons margarine in a skillet over medium heat until tender. Add garlic; saute 1 minute. Stir in spinach, parsley, 1/4 teaspoon salt, pepper, and nutmeg. Remove from heat. Melt 1/3 cup margarine in a saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Add half-and-half and broth; cook over medium heat, stirring constantly, until mixture is thickened. Add lemon juice and remaining 1/2 teaspoon salt.
Spread 1/4 cup sauce in a greased 13- x 9- x 2-inch baking dish. Arrange 3 lasagna noodles over sauce. Layer with one-third of spinach mixture, 1/2 cup Monterey Jack cheese, 1/3 cup Parmesan cheese, 2/3 cup sauce, and 3 lasagna noodles. Repeat layers twice, beginning with spinach mixture. Top with remaining 1 cup sauce, 1/2 cup Monterey Jack cheese, and 1/3 cup Parmesan cheese. Bake, uncovered, at 350° for 30 minutes.